1. Field of the Invention
The present invention relates to a battery charger and a battery charging method that control charging of a plurality of batteries.
Priority is claimed on Japanese Patent Application No. 2008-88723, filed Mar. 28, 2008, the content of which is incorporated herein by reference.
2. Description of Related Art
To independently use loads connected to batteries for each application, battery charging equipment is sometimes configured with a plurality of batteries.
A technology is proposed with regard to a charger that controls charging of a plurality of batteries intended for applications in the above manner (for example, refer to Japanese Unexamined Patent Application, First Publication, No. 2001-128388).
According to this technology, two electrical generators are included, and the voltage of a battery connected via a charger for each of the generators is detected, whereby the batteries can be charged without excess or deficiency.
FIG. 27 is a basic configuration of a conventional battery charger including one electrical generator.
A battery charger 302 includes a circuit that charges two batteries (batteries 3 and 4) by employing a method known as short-circuit method.
The battery charger 302 detects a voltage Vreg of the battery 3 in a charge-state detector 320. When the detected voltage Vreg of the battery 3 exceeds a predetermined set value, a charge controller 330 simultaneously sets three thyristors S11, S12, and S13 of a rectifying processor 340 to an ON state, and short-circuits the output of a 3-phase alternate current (AC) generator 301. The battery charger 302 stops charging the batteries 3 and 4 so that they are not excessively charged by the 3-phase AC generator 301 whose output is short-circuited.
The battery charger 302 repeats a scheme, when the voltage Vreg decreases, the charge-state detector 320 detects reduction in the voltage Vreg, and the charge controller 330 changes the thyristors of the rectifying processor 340 to an OFF state.
A charger that charges using charge voltage control method according to this scheme is termed a short-circuit regulator.
However, in the circuit employing the conventional method according to the technology disclosed in JP 2001-128388 A, while each battery can be charged without excess or deficiency, a plurality of generators must be provided to charge a plurality of batteries. Thereby, the number of constituent components and the scale of the equipment increase. Another problem is that, since the load on an engine for driving the generators also increases, it is not suitable for general use.
As one method of reducing the scale of equipment that is increased by the provision of a plurality of generators, FIG. 27 is a circuit diagram of an arrangement including a single 3-phase AC generator 301. In the battery charger 302 of FIG. 27, a charge-state detector 320 detects voltage Vreg of a battery 3.
According to this method, the charge state is detected based on the voltage Vreg of one battery. Conduction of a thyristor is controlled according to the detected charge state, and both batteries are simultaneously charged or not charged. Therefore, depending on the usage state of an accessory-system load (e.g. a refrigerator, a television, etc.), a battery that is used for the accessory-system load and for which voltage Vreg is not detected is greatly consumed, whereas a battery that is used for the drive-system load and for which voltage Vreg is detected becomes fully charged.
While this method can reduce the scale of the equipment by using only one generator, since it detects the charge state of both batteries by detecting the state of only one battery, it has a problem of being unable to properly manage the amount of charge to both batteries when there is an imbalance in their charge amounts or loads.
Also, the short-circuit regulator described above has a problem that, depending on the state of the connected load, difference may arise in the current flow in each phase of the 3-phase AC generator, causing an imbalance.
Another problem is that, when such a situation arises during high rotation, it is not possible to release an uncharged state.
Another problem is that a short-circuit regulator has electrical loss which considerably affects the drive-system by short-circuit of the output of the generator at low rotation.
Thus, in conventional battery chargers, the discharge amount differs according to the state of the load connected to each battery, whereby, when controlling the charge amount by detecting the voltage of only one of the batteries, a bias is liable to arise in the charge of the batteries according to their load state.